This invention relates to thermocompression bondable protective webs or hot-melt webs which are suited for the protection of members having flexibility, flatness or smoothness depending on the intended application and requiring light transparency, weather resistance and heat resistance or suited for the adhesive joint of components of optical recording media, flat panel displays or the like. It also relates to laminates comprising a member and a hot-melt web laminated thereto and a method for preparing the laminates.
Research efforts have been made on the polymers used as protective members for module sheets requiring light transparency and weather resistance and having in themselves sufficient flexibility to enable winding and unwinding, typically solar cell module sheets.
For example, JP-A 198081/1989 discloses a solar cell module sheet wherein thin film solar cells are formed on a polymer film substrate and covered with a protective film layer with a buffer adhesive film layer of thermoplastic polymer interposed therebetween. This module sheet is an integrated amorphous silicon solar cell array having a sheet thickness of up to 1,000 xcexcm and a flexural rigidity of up to 100 kg.mm2, preferably up to 10 kg.mm2, as measured with a 5-mm wide sample. The module sheet can be wound into a compact roll without damage to the solar cells when unnecessary, and spread flat when necessary. This solar array module sheet is flexible and repeatedly foldable.
The solar array module sheet of JP-A 198081/1989, however, is difficult to repeatedly wind into and unwind from a roll with a diameter of several centimeters because a flexural rigidity up to 100 kg-mm2 is permissible. Although the buffer adhesive film layer of thermoplastic polymer intervenes between the protective film layer and the substrate, this module sheet has several problems. (1) It is less credible in environmental resistance when exposed for a long period of time to sunlight or other light sources during outdoor or indoor use. (2) Simply thermocompression bonding the protective film having a buffer adhesive layer to the thin film solar cell surface does not insure the evenness of the protective film and the transparency of a light receiving surface. (3) In thin film laminate devices including thin film solar cells using a polymer film as the substrate, functional thin films of amorphous silicon, ITO, aluminum alloy, etc. constituting the devices have a higher rigidity than the polymer material and also have different heat shrinkage factors and internal stresses during film formation. As a result, the module sheet undergoes random deformations which are practically impossible to correct.
(4) Bubbles can be introduced during thermocompression bonding. Even minute bubbles can grow into large bubbles with a rise of the service temperature of the device. Such bubble growth detracts from the outer appearance of the device, deteriorates the performance of the solar cells, and causes delamination of the protective film. More particularly, since the patterned surface of solar cells and the surface of thin film modules have numerous irregularities, bubbles are left in the shades of such irregularities after the lamination of the protective film layer. If directly exposed to sunlight, the bubbles expand to larger ones which can scatter the incident light to detract from the power generation capability (voltage-current characteristic or V-I curve), cause delamination and deteriorate the outer appearance. Additionally, moisture in the bubbles can accelerate the deterioration of the initial V-I curve in various environments.
Where a thermoplastic buffer adhesive layer is formed on a protective film as in the above example, the buffer adhesive layer experiences sensitive changes of its dynamic physical properties in response to changes of temperature and humidity in the outdoor or other service environment where the solar panel is used. For example, the light receiving surface is heated by sunlight or radiant heat from a suitable light source. As a result, the buffer adhesive layer lowers its viscosity or softens and fluidizes, detracting from the bond strength between the protective film and the solar cell surface and losing the rubbery elasticity necessary to withstand shrinkage stresses thermally induced in the protective film. This increases the contact resistance with the ITO electrode interface due to elastic deformation of the thin film and delamination of the surface electrode, deteriorates the power transducing capability, and lowers the protective film function causing cell damages. Conversely, at temperatures below the softening point, the thermoplastic buffer adhesive layer lowers its bonding force and hardens, losing the thin film device protecting function as the buffer layer to mitigate stress strain. The protective film function is likely to change in response to humidity changes.
In one example where a polyethylene terephthalate (PET) film having a glass transition temperature (Tg) of about 69xc2x0 C. is used as the above protective film, when the light receiving surface is heated by direct exposure to sunlight, the film experiences substantial dimensional changes due to its thermal shrinkage and expansion. Since the buffer adhesive layer cannot accommodate these deformations, there is a tendency that stress strains are propagated to the solar cells, inducing performance deterioration and protective film delamination. It is then recommended that the device using PET film is limited to the application where the receiving surface avoids continuous direct exposure to sunlight or the indoor application where the receiving surface is exposed to fluorescent lamps and indirect lighting from incandescent lamps.
In thin film multilayer devices using films as the substrate, as typified by solar arrays and flexible printed circuit boards, the film substrates themselves are easy to work, for example, by punching or press working, fine through-hole drilling utilizing a YAG laser, etc., and laser scribing, as compared with glass substrates and metal substrates (e.g., SUS). This is advantageous in that thin film devices of three-dimensionally integrated compact design can be finished, without a need for leads, by drilling through-holes in the rear surface thereof and filling the holes with a conductive paste to form tapping electrodes on the rear surface.
When a process including the step of laying up many functional thin films by a vacuum process, the step of patterning the thin film surface to form a wiring electrode or interlayer insulating film by a screen printing process, and the step of forming a protective layer on the surface is considered with respect to productivity, the use of the substrate in the form of a continuous flexible film capable of bridging the respective steps is advantageous in throughput because the steps can be continuously carried out by virtue of a shorter tact time and the continuous feeding and handling of the substrate. Also, when the large scale integration or large scale mass production of thin film devices is considered, the use of the substrate in the form of a continuous flexible film is advantageous in throughput for the same reason.
Unlike the glass and SUS substrates, however, the flexible film substrates are not constant in xe2x80x9cdeflection,xe2x80x9d xe2x80x9cwrinkle,xe2x80x9d xe2x80x9ccorrugation,xe2x80x9d xe2x80x9cslack,xe2x80x9d and xe2x80x9cthickness uniformityxe2x80x9d when they are processed into thin film devices. Since productivity is lowered by the worsening of these conditions, the flattening and smoothing of flexible film substrates are crucial in order to produce consistent devices while minimizing the productivity lowering.
The manufacture of optical discs such as DVD-R involves the step of joining with a hot-melt or adhesive sheet. The adhesive requires spin coating, screen printing or otherwise coating and UV curing, adding to the number and time of steps. Polycarbonate substrates can be dissolved in or opacified by the solvent used for the coating purpose. Entrainment of bubbles adversely affects the optical properties or exacerbates the outer appearance of products.
Similar problems arise in the manufacture of liquid crystal panel displays, inorganic EL displays, and flat panel displays such as plasma diplay panels (PDP) and ECD. The manufacture process involves the step of laying and bonding functional films and the step of heating and fluidizing a polymer, both for the purposes of increasing luminance, recognition, visibility, environmental resistance, and contrast. In the step of laying and bonding functional films, for example, AR, LR, LG, deflector protective films, high viewing angle, high retardation, polarizing films, phase films, photoconductive diffusion films for back light, reflective multilayer films, transparent film electrodes, and lens sheet retainers and in the step of heating and fluidizing a polymer, for example, the injection of ferroelectric, high molecular weight, high boiling liquid crystals in a cell of a flexible liquid crystal module using a polymer film as the substrate, there arise similar problems.
An object of the invention is to provide a hot-melt web having improved light transparency, heat resistance and weather resistance, allowing less bubbles to be introduced or bubbles to be readily removed during thermocompression bonding, providing a sufficient bonding force, and effectively correcting random deformations of a module sheet; a laminate using the hot-melt web, typically a module sheet; and a method for preparing the laminate.
Another object of the invention is to provide a hot-melt web which is suitable for use in the bonding, joining and laminating steps in the manufacture of optical discs such as DVD and flat panel displays, contains less entrained bubbles, is easy to remove bubbles, and is effective for improving the quality of an associated product; a laminate using the hot-melt web, typically an optical disc or flat panel display; and a method for preparing the laminate.
In a first aspect, the invention provides a hot-melt web or stock comprising a support of a light transmissive, heat resistant resin and a buffer adhesive layer containing a thermosetting resin on at least one surface of the support. In one preferred embodiment, the buffer adhesive layer further contains an organic peroxide. The organic peroxide preferably has a decomposition temperature for the half life of 10 hours of at least 70xc2x0 C. prior to thermocompression bonding. Preferably, the support has a glass transition temperature of at least 65xc2x0 C. or a heat resistant temperature of at least 80xc2x0 C. and a MOR value, representative of a degree of molecular orientation, of 1.0 to 3.0, prior to thermocompression bonding. The hot-melt web is often used for the covering of a member or the adhesion between members to form a laminate. The buffer adhesive layer may be embossed on its surface.
In a second aspect, the invention provides a laminate comprising a member and the hot-melt web laid thereon. The hot-melt web is typically thermocompression bonded to the member. Preferably, the buffer adhesive layer of the hot-melt web has a dynamic modulus of up to 5xc3x97109 dyn/cm2 at 20xc2x0 C. and at least 1xc3x97106 dyn/cm2 at 100xc2x0 C., a maximum peak value of tanxcex4 in a temperature range of up to 20xc2x0 C., and a thickness of 3 to 500 xcexcm subsequent to the thermocompression bonding; and the support of the hot-melt web has a rate of change of its dynamic modulus within 30% at a temperature of 0xc2x0 C. and/or 120xc2x0 C. and a thickness of 5 to 100 xcexcm subsequent to the thermocompression bonding. Also preferably, the member includes a substrate having a MOR value, representative of a degree of molecular orientation, of 1.0 to 3.0 prior to thermocompression bonding. The member is typically an optical disc, flat panel display or solar array.
In a third aspect, the invention provides a method for preparing a laminate comprising the steps of:
passing a hot-melt web and a member through a roll laminator for effecting thermocompression bonding to form a composite laminate, the hot-melt web comprising a buffer adhesive layer on at least one surface of a support of a light transmissive, heat resistant resin, with the buffer adhesive layer being in contact with the member;
cutting the composite laminate into sections, stacking the laminate sections, and placing the stack of laminate sections in a container; and
heating and hydrostatic pressing the stack for achieving thermosetting and bubble removal.
The heating and pressing step preferably uses a heating temperature of at least 70xc2x0 C. and a hydrostatic pressure of 3 to 15 kg/cm2. When the stack of composite laminate sections is heated in the container, a substantially uniform mechanical pressure may be applied to the composite laminate sections in a direction perpendicular to the surfaces thereof, thereby achieving the bubble removal and flattening of the composite laminate sections. More specifically, a mechanical pressure of 0.01 to 5.0 kg/cm2 is applied for flattening the composite laminate sections. Typically, the laminate preparing method of the invention uses the hot-melt web of the first aspect whereby the laminate of the second aspect is obtained.